Jellyfish, corals, and other stingers

Yet, these diverse animals are all
armed with stinging cells called nematocysts. Cnidarians are united
based on the presumption that their nematocysts have been inherited
from a single common ancestor.

The name Cnidaria comes from the Greek word "cnidos," which means stinging nettle.
Casually touching many cnidarians will make it clear how they got their name when their
nematocysts eject barbed threads tipped with poison.

Many thousands of cnidarian species live in the world's oceans, from the
tropics to the poles, from the surface to the bottom. Some even burrow.
A smaller number of species are found in rivers and fresh water lakes.

There are four major groups of cnidarians:Anthozoa, which includes true corals, anemones, and sea pens;Cubozoa, the amazing box jellies with complex eyes and potent toxins;Hydrozoa, the most diverse group with siphonophores, hydroids,
fire corals, and many medusae; andScyphozoa, the true jellyfish.